Traveling wave tube utilizing a secondary emissive cathode



y 3 w. A. SMITH, JR., ETAL 3,096,457

TRAVELING WAVE TUBE UTILIZING A SECONDARY EMISSIVE CATHODE Filed March 51, 1959 2 Sheets-Sheet 1 F/G. I

//VVWTOR WILLIAM A. SMITH JR. GEORGE h. MAC/"ASTER WWM A T TORNE Y y 1963 w. A. SMITH, JR.. ETAL 3,096,457

TRAVELING WAVE TUBE UTILIZING A SECONDARY EMISSIVE CATHODE Filed March 51, 1959 2 Sheets-Sheet 2 RR Y m M N R m m V WM T An A M mm% n r wua United States Patent TRAVELING WAVE TUBE UTILIZING A SECOND- ARY EMISSEVE CATHUDE William A. Smith, Jr., Milton, and George H. Mach/faster, Waltham, Mass, assignors to Raytheon Company, a

corporation of Delaware Filed Mar. 31, 1959, Ser. No. 803,326 7 Claims. (Cl. 313103) This invention relates to traveling wave devices receptive of high frequency input energy having cathodes which are activated solely through secondary emission processes.

As is Well known, traveling wave tubes make use of the interaction between an electron beam launched from a primarily emissive cathode which is projected along a slow Wave propagating structure and the electromagnetic held of the wave energy guided by said structure. The fields of wave energy of such periodic propagating structures may be resolved into a series of so-called space harmonic waves which travel with different phase velocities; the electron beam can be made to interact with one of the components of the electromagnetic field by varying the electron velocity so that it is in substantial synchronism with the phase velocity of that space harmonic wave component. In the case of some of the space harmonies, the phase and group velocities are oppositely directed and these harmonics are known as backward waves. Other harmonics, called forward waves, are characterized in that the phase and group velocities are in the same direction. Amplification and generation of oscillations may be obtained by interaction between an electron beam and either forward Waves or backward waves. The invention is applicable to the so called M-type traveling wave tubes which are characterized in that the electron beam moves through mutually perpendicular unidirectional electric and magnetic fields, and is applicable to traveling wave amplifier tubes or to locked oscillator tubes, either of the backward wave or forward wave type.

M-type traveling wave tubes of the prior art require a heater and heater circuitry for efiecting thermionic emission from the cathode, as Well as a source of energy for activating the heater. During operation of such traveling wave tubes, considerable emissive material is evaporated from the cathode surface, resulting not only in eventual loss of primary emission capability, but also in undesirable deposition of evaporated material upon the slow wave structure and the output surfaces of the tube. The requirement of means for directly or indirectly heating the emissive surfaces of conventional hot cathodes in M-type tubes introduces problems in cathode heater design which may be relatively difficult to solve, particularly when the tube is subjected to considerable mechanical shock and vibration. Satisfactory shockproof mounting of a heater becomes increasingly difficult as the length of the heater increases; this would be particularly true in the case of linear tubes of the traveling wave type having a cathode which is coextensive with an appreciable por-. tion of the length of the slow wave structure. Furthermore, heater circuitry is needed in tubes of the prior art to supply the necessary heater operating potentials; consequently additional power supply requirements, as well as additional electrical hardware, is needed.

' 'In accordance with this invention, the traveling wave tube is provided wtih a cathode wherein electrons are emitted, either solely or principally, by means of secondary emission therefrom. This invention is applicable to a highly evacuated traveling Wave tube which (1) inherently contains provision for supplying higher frequency input energy, which (2) depends upon interaction between high frequency wave energy and an electron beam moving in mutually perpendicularly unidirectional electric and magnetic fields, and which (3) contains a sufficient number of gas molecules in the tube interaction space to achieve the necessary secondary emission of electrons to satisfy the requirements of the particular tube. The latter requirement is readily met, since there are normally sufficient gas particles, even in relatively high vacuum tubes, to initiate secondary emission of sufiicient degree to insure proper tube operation. For example, satisfactory operation has been obtained within a range of pressures from about 10** mm. of mercury to about 10" mm. of mercury.

Several types of cold cathodes may be used successfully; examples include tungsten-thcria cermet cathodes, oxide-coated cathodes, nickel rods, which may be platinum-coated, and copper rods. The tungsten-thoria cermet cathode may, for example, comprise tungsten and 10% thor-ia or 70% tungsten and 30% thoria. This list is illustrative rather than exhaustive. Most metals have a secondary emission ratio lying between about 1 and 1.8 and most of these metals would be suitable for use as a cathode, from this standpoint alone. The cathode material must, of course, be such that it does not undergophysical or chemical deterioration over [the operating temperature range of the cathode. :For example, a material whose melt-ing point is less than the maximum operating temperature of the cathode obviously Wolud be unsatisfactory. It is further necessary to use a cathode of relatively low vapor pressure; otherwise, problems inherent in gassy tubes arise, such as severe high frequency energy losses.

Another requirement for the cathode is that it have a sufiiciently high emission current density; some metals are relatively poor secondary emitters and would be unable to provide the requisite current per unit of cathode surface area needed in certain tube applications.

When high frequency energy is supplied to the tube, the free electrons and gas particles present in the tube are caused to bombard the cathode. Since the cathode is seeondarily emissive, electrons leave the cathode surface owing to the initial bombardment; this process is cumulative up to a point at which satisfactory emission occurs.

The advantages of the traveling wave tube according to the invention are several. No direct or indirect means of heating the electron-emitting surface is required to initiate the electron beam. Consequently, tube construction, as well as the tube power supply, is simplified appreciably. Elimination of the heater permits one to design a given tube which has increased mechanical reliability and stability. High resistance to mechanical shock and vibrationso important in tubes such as those destined for airborne installations-is much more easily achieved With a tube according to the invention than with traveling wave tubes requiring cathode heaters; 21 large part of conventional tube failures result from physical damage to the heater assembly. The emitting surface of the cathode in tubes according to the invention is much cooler during operation than that of conventional traveling Wave tubes. Therefore, the rate of evaporation of emitting material from the cathode surfacewhich evaporation eventually constitutes one of the major causes of tube failure-is considerably reduced. Moreover, the problem of heater burnout, resulting Whenever the cross-section of the heater coil has been reduced sufficiently by evaporation of metal from the heater coil, is prevented. Furthermore, deterioration of tube performance resulting from deposition of evaporated material upon the slow wave structure and output surfaces of the tube is minimized.

No current can be drawn from the tube according to the invention, with operating voltages applied, until the high .9 frequency driving energy is supplied. Ihis characteristic is useful, for example, in triggering pulsed amplifier chains.

Further objects and features of this invention will be understood more clearly and fully from the following detailed description of the invention with reference to the accompanying drawings, wherein:

FIG. 1 is a central cross-section view of a traveling wave tube in accordance with the invention; and

FIG. 2 is a View illustrating the manner in which external connections are made to the tube of FIG. 1.

Referring now to the drawing, a typical traveling wave tube of cylindrical configuration is indicated by the reference numeral and includes a slow wave propagating structure 12, a cathode assembly 14 arranged substantially concentric with the slow wave structure parallel to the longitudinal axis of the tube, a magnet assembly 16 and external energy coupling means 18.

The traveling wave tube 10 includes a cylindrical envelope member 21, an upper cover plate 22 and a lower cover plate 23, which combine to form a portion of an evacuated tube envelope.

The slow wave structure 12 includes a plurality of elements 25 which may be formed of rods 26 having plates 27 brazed or otherwise afiixed to the inner periphery thereof. The elements 25 are circularly arranged within cylindrical member 21 and the plates 27 are radially disposed, in the manner of conventional magnetron anode vanes. The slow wave structure 12 shown in FIG. 1 is a typical structure and the invention is not limited thereto; other types of slow wave structures are indicated in a co-pending application, Serial No. 694,698, of William A. Smith, Jr., filed October 30, 1957. Although not indicated in the drawing, the rods 26 may be made hollow to permit passage of cooling fluids; the tube 10 then would have to include inlet and outlet fluid conduits. A pair of straps 31 and 32 connect with alternate elements 25, as indicated clearly in FIG. 2. Since the straps are broken, the slow wave structure 12 is electrically non-reentrant, that is, it has two ends whose locations correspond to those of the ends of each strap.

The magnet assembly 16 includes a pair of pole pieces 35 and 36 to which are attached C-shaped magnets 37 and 38. The pole pieces are disposed so that the magnetic flux is concentrated in the interaction space 39 between the active portion of the cathode l4 and the slow wave structure 12.

The cathode assembly 14 of FIG. 1, which may be used with a cathode of relatively high secondary emission ratio, includes a sleeve assembly 41 having a central portion 42 which may be coated, for example, with an oxide of an electron-emissive material. The sleeve assembly 41 is provided with conventional end shields 43 and 44. The cathode sleeve assembly 41 is bonded to an elongated rod or stem 45 which extends outside the evacuated tube envelope. The cathode assembly 14 is mechanically supported from upper pole piece 35 of the magnet assembly 16 by means of an electrically insulating bushing 47. One end of bushing 47, which may be made of ceramic, is afiixed to the pole piece 35, while the other end of the bushing is hermetically sealed to the cathode stem 45 by means of an apertured discoidal member 48. In many applications, particularly where cathodes of lower secondary emission are adequate, the cathode assembly 14 may comprise a single elongated rod whose configuration is like that of stem 45. To facilitate connection of a cathode lead to the cathode, an end cap 49 is secured, as by brazing, to one end of the cathode stem 45.

A unidirectional electric field is produced between the cathode sleeve assembly 41 and the slow wave structure 12 by means of a battery 51 or other unidirectional voltage source. This electric field is directed perpendicularly to the magnetic field existing between the pole pieces 35 and 36, so that electrons from the cathode are subjected to crossed electric and magnetic fields.

I An aperture is provided in the envelope member 21 of tube it) through which two parallel wire lines 54 and 55 may be brought inside the tube envelope. The input parallel wire line 54 consists of two parallel electrical conductors 56 and 57, while output parallel wire line 55 consists of two parallel conductors 58 and 59. A double ridge wave guide 64 includes a first or input section 61 and a second or output section 62 electrically isolated from the input section 61 by an electrically conductive partition or septum 63. The ends of the wave guide sections 61 and 62 remote from the septum are adapted to be connected to various input and output circuit components, such as a source of energy and a load. Output wave guide section 62 includes a pair of juxtaposed ridges 65 and 66 which may be secured, as by screws 67, to opposite walls of the wave guide. Similarly, input wave guide section 61 includes a pair of ridges 68 and 69 secured to opposite walls of wave guide 66. These ridges may be tapered exponentially at the ends remote from septum 63 to provide proper impedance matching between wave guide 6%) and the corresponding parallel wire line. The conductors 5:3 and 59 of output transmission line 55 interconnect the ridges 65 and 66 of the wave guide 649 and one end of the strap rings 31 and 32, all respectively. Likewise, the conductors 56 and 57 of parallel wire line 54 interconnect the ridges 68 and 69 of the input section 61 of wave guide 66 and the other end of the strap rings 31 and 32, all respectively. The wires of each transmission line 54 and 55 should not be too close to the partition 63 lest the latter effect the field existing about the transmission lines. Details and design considerations of the external coupling arrangement 1%, including other means of impedance matching the wave guide sections to the parallel wire lines, may be found in the aforesaid co-pending application.

Energy from a driving source, not shown, may be supplied to the input wave guide section 61 in the direction of the arrow in FIG. 2. This input energy passes through Wave guide section 61, input transmission line 54, traveling wave tube It}, output transmission line 55, output wave guide section 62, and thence to a load (not shown), in the order named.

This invention is also applicable to locked oscillators, that is, oscillators having a frequency locking signal applied to an input terminal; such locked oscillators are shown and described in United States Letters Patent 2,888,649, of Edward C. Dench and Albert D. La Rue, issued May 26, 1959. When used as a locked oscillator, the locking signal is supplied to input section 61 in lieu of the input signal to be amplified in FIG. 2, and the oscillatory energy generated in the tube It) would then be removed from the output section 62 of wave guide 60.

It should be noted that either end of the slow wave structure 12, whether the device is used as an amplifier or as a locked oscillator, may be the input end; the other end of the structure 14 then would be the output end. Likewise, the portion 61 of Wave guide 60 may be the output end and the portion 62 of wave guide 66 the input end. If the device is to operate as a backward wave device, the output coupling means would be disposed at the upstream end of the slow wave structure, that is, at the end of the slow wave structure away from which electrons move. On the other hand, if the device is to operate as a forward Wave amplifier, the output coupling means would be disposed at the downstream end of the slow wave structure, that is, at the end of the slow wave structure toward which electrons move.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. A traveling wave electron discharge device comprising a delay line for propagating a traveling wave, an

input terminal coupled to one end of said delay line, a cold cathode, and means for supplying high frequency input energy to said input terminal for effecting initial bombardment of said cold cathode by free electrons and by gas molecules, said cathode supplying electrons for injection along paths adjacent said delay line in energyexchanging relation with said traveling wave.

2. A traveling Wave electron discharge device compris ing a delay line for propagating a traveling wave, an input terminal coupled to one end of said delay line, a cold cathode, and means for supplying high frequency input energy to said input terminal for efiecting initial bombardment of said cold cathode by free electrons and by gas molecules, said cathode supplying electrons substantially entirely by secondary emission for injection along paths adjacent said delay line in energy-exchanging relation with said traveling wave.

3. A traveling Wave electron discharge device comprising a nonreentrant delay line for propagating a traveling Wave, an input terminal coupled to one end of said delay line, a cold cathode, and means for supplying high frequency input energy to said input terminal for effecting initial bombardment of said cold cathode by free electrons and by gas molecules, said cathode supplying electrons substantially entirely by secondary emission for injection along paths adjacent said delay line in energy-exchanging relation with said traveling wave.

4. A traveling Wave electron discharge device comprising a nonreentrant delay line for propagating a traveling wave, an input terminal coupled to one end of said delay line, a cold cathode, and means for supplying high frequency input energy to said input terminal for effecting initial bombardment of said cold cathode by free electrons and by gas molecules, said cathode supplying electrons substantially entirely by secondary emission for injection along paths adjacent said delay line in energy-exchanging relation with said traveling wave, said electrons making more than one traversal past a given point on said delay line in the same direction.

5. A traveling wave electron discharge device comprising a nonreentrant delay line for propagating a traveling wave, an input terminal coupled to one end of said delay line, a cold cathode, means for supplying high frequency input energy to said input terminal for effecting initial bombardment of said cold cathode by free electrons and by gas molecules, means for producing an electric field between said delay line and said cathode, and means for developing a magnetic field perpendicular to said electric field, said cathode supplying electrons substantially entirely by secondary emission for injection along paths adjacent said delay line perpendicular to said electric and magnetic fields and in energy-exchanging relation with said traveling wave, said electrons making more than one traversal past a given point on said delay line in the same direction.

6. A traveling Wave electron discharge device comprising a non'reentrant delay line for propagating a traveling wave, an input terminal coupled to one end of said delay line, a cold cathode, means for supplying high frequency input energy to said input terminal for effecting initial bombardment of said cold cathode by free electrons and by gas molecules, means for producing an electric field between said delay line and said cathode, and means for developing a magnetic field perpendicular to said electric field, said cathode supplying electrons substantially entirely by secondary emission for injection along paths adjacent said delay line perpendicular to said electric and magnetic fields and in energy-exchanging relation with said traveling wave.

7. A traveling wave electron discharge device comprising a nonreentranit delay line for propagating a traveling Wave, an input terminal and an output terminal coupled to opposite ends of said delay line, a cold cathode, means for supplying high frequency input energy to said input terminal for efiecting initial bombardment of said cold cathode by free electrons and by gas molecules, means for producing an electric field between said delay line and said cathode, and means for developing a magnetic field perpendicular to said electric field, said cathode supplying electrons substantially entirely by secondary emission for injection along paths adjacent said delay line perpendicular to said electric and magnetic fields and in energy-exchanging relation with said traveling wave, said electrons making more than one traversal past a given point on said delay line in the same direction.

References Cited in the file of this patent UNITED STATES PATENTS 2,244,318 Skellett June 3, 1941 2,409,038 Hansell Oct. 8, 1946 2,427,781 Hansell Sept. 23, 1947 2,438,194 Steele et a1. Mar. 23, "1948 2,448,527 Hansell Sept. 7, 1948 2,450,763 McNall Oct. 5, 1948 2,673,306 Brown Mar. 23, 1954 

1. A TRAVELLING WAVE ELECTRON DISCHARGE DEVICE COMPRISING A DELAY LINE FOR PROPAGATING A TRAVELING WAVE, AN INPUT TERMINAL COUPLED TO ONE END OF SAID DELAY LINE, A COLD CATHODE, AND MEANS FOR SUPPLYING HIGH FREQUENCY INPUT ENERGY TO SAID INPUT TERMINAL FOR EFFECTING INTIAL BOMBARDMENT OF SAID COLD CATHODE BY FREE ELECTRONS AND BY GAS MOLECULES, SAID CATHODE SUPPLYING ELECTRONS FOR 